Earthquakes in Maine

Henry N. Berry IV

Maine Geological Survey
Department of Conservation

Seismic Activity

Seismic activity in Maine is typical of the Appalachian region of northeastern North America. There is a low but steady rate of earthquake occurrence. The earthquakes are presumably caused by modern stress being released occasionally along zones of weakness in the earth's crust, but a more specific cause for the earthquake activity is not known. Recorded earthquake locations and detailed seismic motion studies do not show any clear correlation with either local or regional geologic features (Ebel, 1989). No significant amount of motion has been shown for any fault since the last Ice Age, about 20,000 years ago, and geologic evidence demonstrates that many faults have been inactive since the formation of the Appalachians, over 300,000,000 years ago. None of the ancient faults in Maine have been identified as active (Anderson et al., 1989; Osberg et al., 1989).

The largest earthquake recorded in Maine between 1747 and 1992 was near Eastport in 1904 with a Modified Mercalli intensity estimated at VII. The largest accurate measurement was on June 15, 1973 from an earthquake just on the Quebec side of the border from northern Oxford County, Maine, with a Richter magnitude of 4.8 (Johnston, 1995). Most Maine earthquakes are of small magnitude. Many are too small to feel. No Maine earthquake has caused significant damage. The persistent activity, however, indicates that some crustal deformation is occurring and that a larger earthquake cannot be ruled out (Ebel, 1989). Based on past earthquake data collected over a limited time span (1975-1982) from the whole of New England, and assuming that Maine is a representative part thereof, Ebel (1984) has estimated the return times for earthquakes in Maine as follows:

Table 1. Estimated return times for earthquakes of different magnitudes in Maine.

Magnitude

4.6

5.0

5.5

6.0

6.5

7.0

Return time (years)

24

52

138

363

955

2512

(±20 to 30%)

Ebel (1984) also made similar estimates based just on the seismic activity for Maine recorded in that same time span, and arrived at slightly shorter return times for magnitude 5.0 or smaller, and slightly longer return times for magnitude 6.5 or greater earthquakes.

Geographic Distribution

Earthquakes have been reported from all counties in Maine, thereby indicating some level of statewide exposure. The occurrence is not equally distributed, however, and both modern and historical records indicate somewhat higher activity in the eastern, central, and southwestern parts of the State (Figure 1).

Figure 1. Map showing interpreted areas that have experienced somewhat higher earthquake activity in historical times (1747-1975) and in recent times (1975-1982). (modified from Ebel, 1989)

The amount of direct physical damage from an earthquake depends on a combination of factors, including the earthquake intensity, the density of structures in proximity to the epicenter, stability during an earthquake of geologic materials beneath structures, and the construction features of structures exposed to seismic vibration. In general, earthquakes with magnitudes less than 5.0 (corresponding approximately to Modified Mercalli intensity VI) cause little or very localized damage (Johnston, 1995). The largest reported Maine earthquakes have caused some damage to property near the epicenter, such as damaging chimneys and breaking glass (Ebel, 1989).

Most Maine buildings were not constructed to withstand the lateral motion of a significant (magnitude 6 or higher) earthquake. In general, unreinforced brick and masonry structures are especially prone to earthquake damage. Coastal and lakefront structures built on water-saturated, unconsolidated material such as artificial fill, may be vulnerable to liquefaction in a severe earthquake. Liquefaction is a loss of cohesion between particles due to lubrication by water during vibration, causing a sudden loss of strength. Most death and injury during earthquakes results from people being struck or trapped by falling debris (NESEC, 1993). Other possible concerns in a severe earthquake emergency would be the disruption of infrastructure facilities, such as road access, gas and oil pipelines, and electricity and water supplies, and the disruption of emergency services such as police, firefighting, ambulance, and hospital services (Ludman and Coch, 1982).

History

Instrumental monitoring of seismicity in Maine began in 1975, marking the beginning of the modern record. Earthquake events prior to 1975 are considered part of the historical record. The record of earthquake activity prior to the mid-1800's is sparse and of uneven quality (Ebel, 1989). The history of earthquake events, therefore, is well known only for the very recent part of the geologic past. The record of 507 historical and modern earthquake events including dates, epicenter locations, and sizes has been compiled by the Maine Geological Survey for the years 1747 to 1992 and is available in an open-file map format (Johnston, 1995). These data show that a higher incidence of modern earthquake activity has occurred in areas with a higher incidence of historical activity (Figure 1). The areas of relatively higher seismic activity include the Passamaquoddy Bay area of eastern Washington County; the Dover-Foxcroft - Milo area of southern Piscataquis County; and southwestern Maine in general, particularly the Portland-Lewiston region of Androscoggin and Cumberland Counties. The boundaries of these areas of higher activity are not clearly defined, and minor earthquake activity has been scattered across most areas of the State (Figure 2).

Figure 2. Estimated or measured epicenter locations of all 507 earthquakes recorded in Maine from August, 1747, to January, 1992 (from Johnston, 1995).

The largest earthquakes recorded in Maine are from the historical record. For most of them the magnitude was not measured, so an estimated intensity is assigned on the basis of the reported effects. Relative intensity is measured by the Modified Mercalli scale in which intensities I, II, and III are not felt by most people; intensity IV (noticeable) causes dishes to rattle; intensity V (moderate) is felt by most people and may cause plaster or windows to crack; intensity VI (rather strong) may move heavy furniture and cause some chimney damage; intensity VII (strong) causes considerable damage to poor structures and slight to moderate damage to ordinary structures; and intensity VIII (very strong) causes considerable damage and partial collapse of ordinary buildings. The scale continues through intensities IX to XII, but no earthquakes of this size have ever been reported for the New England region.

Table 2. Maine earthquakes from the historical record (1800-1975) with intensity VI or greater (from Ebel, 1989). Intensities given are the maximum values for each event using the Modified Mercalli scale.

Date

Place

Intensity

Comments

1904

Eastport area

VII

Largest earthquake in Maine (Reid, 1911)

1869

Passamaquoddy Bay?

VI

Location of epicenter uncertain, may have been in Bay of Fundy or in Eastport area.

1857

Lewiston

VI

1905

Sabattus

VI

1912

Eastport

VI

1918

Bridgton/Norway

VI

magnitude 4.5

1928

Milo

VI

magnitude 4.5

1957

Portland

VI

magnitude 4.2

1973

Bowmantown Twp.

VI

magnitude 4.8

The most significant Maine earthquakes in the modern record have not caused much damage, but have been widely felt throughout the State. These are listed in Table 3.

Table 3. Significant Maine earthquakes from the modern record (1975-1992). Data are from the New England Seismic Network operated jointly by Weston Observatory of Boston College and MIT. Magnitudes listed (Mc) are based on the duration of earthquake coda waves. Intensity is the maximum Modified Mercalli intensity (Ebel, 1989).

Date (local time)

Place

Magnitude

Intensity

April 17, 1979

Bath

4.0

V

May 29,1983

Dixfield

3.9

V

Jan. 19, 1984

Machias

3.8

IV

Dec. 28, 1988

Albion

4.0

IV

Sept. 15, 1994

Springfield

3.9

IV

Feb. 25, 1999

Winslow-China

3.7

There have been some larger earthquakes with epicenters elsewhere in New England and neighboring Canada that have been felt in Maine. The most notable of these was the earthquake of November 18, 1755 with an epicenter offshore from Cape Ann, Massachusetts. This earthquake is estimated to have had magnitude 6.0 and affected Cape Ann with intensity VII and southern Maine with intensity VI. The Cape Ann earthquake was notable because it affected a populated area. The St. Lawrence Valley northeast of Quebec City is perhaps the most active area in northeastern North America, but has a low population density. Other significant earthquakes in the surrounding region that have affected Maine are listed in Table 4.

Table 4. Some significant earthquakes in the area surrounding Maine. Magnitudes have been estimated for earthquakes that were not measured. Intensities listed are the maximum intensities, in the epicentral region. Intensities generally decrease with distance from the epicenter.

Date

Place

Magnitude

Maximum
intensity

Comments

1755

Cape Ann, Massachusetts

6.0?

VIII

toppled chimneys in Boston

1925

La Malbaie, Quebec

6.4-6.6?

IX

90 miles from Quebec City. Damaged some types of stone and brick walls over 100 miles away (Hodgson, 1950).

1935

Temiscaming, Quebec

6.2

VII

1940

Ossipee, N.H. (2 events)

5.5, 5.5

VII

some chimneys in Augusta cracked

1982

Miramichi, New Brunswick

5.7

VII

felt across Maine

1988

Chicoutimi, Quebec

6.0

VIII

felt in New York City. Largest in eastern North America since 1935.

1997

Quebec City

5.1

VII

felt across Maine

Monitoring

The Maine Geological Survey (MGS) routinely maps, interprets, and publishes basic bedrock, surficial, and marine geologic information for Maine. The MGS provides advisory and interpretive information on earthquakes for planning and regulatory agencies. After an eathquake event, the MGS collects information from people in the area through an earthquake questionnaire made available to the general public and to county emergency management agencies.

The New England Seismic Network, operated by the Weston Observatory of Boston College, maintains a network of seismic stations across New England that monitors, analyzes, and reports earthquake activity in Maine.